The central nervous system (CNS) is comprised of the spinal cord, brain and retina, and contains trillions of nerve cells (neurons) that form networks capable of performing exceedingly complex functions. CNS neurons require energy to survive and perform their physiological functions, and it is generally recognized that the only source of energy for CNS neurons is the glucose and oxygen delivered by the blood. If the blood supply to all or any portion of the CNS is shut off, thereby depriving neurons of both oxygen and glucose (a condition known as ischemia), the deprived neurons rapidly degenerate. This condition of inadequate blood flow is commonly known in clinical neurology as a “stroke.” If only the oxygen supply to the brain is interrupted, for example in asphyxia, suffocation or drowning, the condition is referred to as “hypoxia”. If only the glucose supply is disrupted, for example when a diabetic takes too much insulin, the condition is called “hypoglycemia”. All of these conditions involve energy deficiency and are recognized in clinical medicine as potential causes of brain damage. In the following text, the terms “energy deficiency” or “ischemia” are used interchangeably to refer to any of these conditions that entail CNS energy impairment.
In recent years, neuroscientists have made considerable progress in understanding the mechanism by which energy deficiency leads to neuronal degeneration. It has been learned that glutamate, which functions under normal and healthy conditions as an important excitatory neurotransmitter in the CNS, can exert neurotoxic properties referred to as “excitotoxicity” if ischemic conditions arise. Normally, glutamate is confined intracellularly, and is only released from a nerve cell at a synaptic junction in tiny amounts, for purposes of contacting a glutamate receptor on an adjacent neuron; this transmits a nerve signal to the receptor-bearing cell. Under healthy conditions, the glutamate released into the extracellular fluid in a synaptic junction is transported back inside a neuron within a few milliseconds, by a highly efficient transport process.
The excitotoxic potential of glutamate is held in check as long as the transport process is functioning properly. However, this transport process is energy dependent, so under ischemic conditions (energy deficiency), glutamate transport becomes incompetent, and glutamate molecules which have been released for transmitter purposes accumulate in the extracellular synaptic fluid. This brings glutamate continually in contact with its excitatory receptors, causing these receptors to be excessively stimulated, a situation which can literally cause neurons to be excited to death. Two additional factors complicate and make matters worse: (1) overstimulated neurons begin to release excessive quantities of glutamate at additional synaptic junctions; this causes even more neurons to become overstimulated, drawing them into a neurotoxic cascade that reaches beyond the initial zone of ischemia; and, (2) overstimulated neurons begin utilizing any available supplies of glucose or oxygen even faster than normal, which leads to accelerated depletion of these limited energy resources and further impairment of the glutamate transport process. Thus, energy deficiency conditions such as stroke, cardiac arrest, asphyxia, hypoxia or hypoglycemia cause brain damage by a compound mechanism; the initial causative mechanism is the ischemia itself, but this leads to failure of the glutamate transport system and a cascade of glutamate-mediated excitotoxic events that are largely responsible for the ensuing brain damage.
In addition to the conditions already mentioned, it has recently become recognized that various defects in the neuron's ability to utilize energy substrates (glucose and oxygen) to maintain its energy levels can also trigger an excitotoxic process leading to death of neurons. It has been postulated that this is the mechanism by which neuronal degeneration occurs in neurological diseases such as Alzheimer's dementia, Parkinsonism, Huntington's Chorea and amyotrophic lateral sclerosis. For example, evidence for defective intracellular energy metabolism has been found in samples of tissue removed by biopsy from the brains of patients with Alzheimer's disease and this has been proposed as the causative mechanism that triggers an unleashing of the excitotoxic potential of glutamate with death of neurons in Alzheimer's disease thereby being explained by an energy-linked excitotoxic process. Evidence for an intrinsic defect in intracellular energy metabolism has also been reported in Parkinsonism and Huntington's Chorea. Thus, rational therapeutic strategies for preventing neuronal degeneration in these disorders would include methods that correct energy deficiency or that prevent excitotoxic neuronal degeneration.
Neurodegenerative diseases are a group of disorders characterized by changes in the normal neuronal function, leading, in most cases, to neuronal death (most of these diseases are associated, especially in late stages, with severe neuronal loss). In most instances, the etiological causes are unknown and they have a progressive development. The end point of neurodegenerative diseases, without exception, extracts an enormous emotional, physical and financial strain on the affected individual and wider community.
The most consistent risk factor for developing a neurodegenerative disorder, especially Alzheimer's disease or Parkinson's disease, is increasing age. Over the past century, the growth rate of the population aged 65 and beyond in industrialized countries has far exceeded that of the population as a whole. Thus, it can be anticipated that, over the next generations, the proportion of elderly citizens will double, and, with this, possibly the proportion of persons suffering from some kind of neurodegenerative disorder. This prediction is at the center of growing concerns in the medical community and among lawmakers, for one can easily foresee the increasing magnitude of emotional, physical, and financial burdens on patients, caregivers, and society that are related to these disabling illnesses. Compounding the problem is the fact that while, to date, several approved drugs do, to some extent, alleviate symptoms of several neurodegenerative diseases, their chronic use is often associated with debilitating side effects, and none seems to stop the progression of the degenerative process. In keeping with this, the development of effective preventive or protective therapies has been impeded by the limitations of our knowledge of the causes and the mechanisms by which neurons die in neurodegenerative diseases. Despite this bleak outlook, several neurobiological breakthroughs have brought closer than ever the day when the secrets of several neurodegenerative disorders will be unlocked and effective therapeutic strategies will become available.
Significant advances have been made in developing methods for preventing or reducing the neuronal damage associated with CNS ischemia. The most active research in this area involves methods of inhibiting excitatory activity at glutamate receptors, using receptor-specific antagonist drugs (in pharmaceutical terminology, a drug that occupies and blocks a certain receptor on a cell surface without triggering activity at that receptor is called an antagonist of that receptor). The glutamate receptors that can mediate excitotoxic neuronal degeneration are broadly divided in two broad categories designated as “NMDA” and “non-NMDA” receptors. NMDA receptors are named after N-methyl-D-aspartate, a drug which does not naturally occur inside the brain, but which was discovered to bind strongly to certain glutamate receptors, which were therefore called “NMDA receptors.” The “non-NMDA” class of glutamate receptors has more recently been subdivided into two distinct categories, referred to as KA (kainic acid) receptors and AMPA receptors (formerly called QUIS receptors).
It has been demonstrated repeatedly that NMDA receptor antagonists can protect against CNS ischemic neuronal degeneration in both in vitro tests and a number of in vivo animal models; however, various items of more recent evidence suggest that NMDA antagonists may be ineffective in one major type of ischemia known as “global ischemia” and provide only partial protection in the other major type of ischemia, known as “focal” ischemia. Moreover, it appears that NMDA antagonists must be administered immediately at the onset of ischemia to provide significant protection. Experimental evidence pertaining to non-NMDA antagonists is more limited, but the few in vivo animal studies available suggest that these agents may provide significant protection against ischemic neuronal degeneration, even when applied after the ischemic event.
Despite claims that either NMDA or non-NMDA antagonists, used alone, can provide substantial protection against CNS ischemia, a growing body of evidence suggests that the degree of protection afforded by either NMDA or non-NMDA antagonists, alone, is relatively modest
A significant limitation of glutamate receptor antagonists as neuroprotectants against ischemic neurodegeneration is that they only insulate the neuron temporarily against degeneration; they do not do anything to correct the energy deficit, or to correct other derangements that occur secondary to the energy deficit. Therefore, although these agents do provide some level of protection against ischemic neurodegeneration in experimental animal models, the protection is only partial and in some cases may only be a delay in the time of onset of degeneration, as mentioned above. However, it is important to note that a delay in the onset of degeneration may be extremely valuable, if there are other drugs or procedures that can be applied during the delay interval to provide additional and/or lasting protection.
One critical factor which is not adequately addressed in most ischemia research concerns the timing of drug administration in relation to the injurious (ischemic) event. This is an important consideration; although some ischemic events can be predicted (for example, involving open-heart surgery), the great majority cannot, and in most cases, therapy can only be initiated during or after an ischemic event. Since CNS cells begin to degenerate very rapidly after the onset of ischemia, there is clearly a need for new neuroprotective methods that are effective when applied after CNS neurons have begun to degenerate.
Another important consideration is whether the ischemia is only transient (e.g. during an episode of cardiac arrest) or is permanent (e.g. following thrombotic or embolic occlusion of CNS blood vessels). If the ischemia is transient, the blood supply carrying oxygen and glucose to the CNS is restored immediately after the event and drugs that prevent neuronal degeneration or promote recovery from the ischemic insult can reach the ischemic tissue through the blood circulation.
If the blood supply to a region of the brain is permanently blocked by a clot, it is not possible by current methods to prevent neuronal degeneration in the center of the ischemic area, because the ischemic tissue is permanently deprived of oxygen and glucose and drugs cannot be delivered to the ischemic tissue through the blocked blood vessel. However, there is a large tissue zone, known as the penumbra, at the circumferential margin of the ischemic area which receives blood from adjoining CNS regions, and this tissue zone is a potential target for drug therapy. Also, drugs that dissolve blood clots (thrombolytic agents, such as streptokinase and tissue plasminogen activator), which currently are being used to treat heart attack victims, are being tested and developed for restoring blood supply to the CNS after a stroke. When such drugs become widely available for CNS use in humans, it will be possible to use them to open the blood vessel so that the ischemic CNS tissue can receive not only oxygen and glucose but also the drugs disclosed herein which can prevent neuronal degeneration or promote recovery from the ischemic insult.
Finally, there are special situations such as thrombotic occlusion of the major artery supplying blood to the retina of the eye, which can be aided by the drugs disclosed herein. When this blood vessel is occluded, it is possible to deliver the drugs of this invention to the ischemic retina by injecting the drug directly into the vitreous of the eye (i.e., into the clear fluid inside the eyeball). The drug can rapidly diffuse from the vitreous into the retina.
The development of therapeutic agents capable of preventing or treating the disease/disorder consequences of ischemic events, whether acute or chronic, would be highly desirable.